This bug appears to be present in both v0.58.1 and again in the latest 7/30/2017 build but didn't appear to be a problem in the v0.60.1 beta from 12/11/2016.

In short, D1 Drillers in D2X-Rebirth will drop shields sometimes when they are destroyed (and other times they will drop Vulcan Ammo sometimes if you don't already have max Vulcan rounds). Of course, some of the time they drop nothing at all too (which is correct). In Dos, with many trials, these D1 Drillers always drop either Vulcan Ammo (there's a chance of them dropping them only if you're not already full) or nothing. They never drop shields. In fact, no stock D1 or D2 robot that fires either Vulcan or Gauss is supposed to ever drop energy or shields.

You may want to go into DLE and remove the HAM file within the HOG (it contains custom weapons I do not use), but I don't think you have to. This issue occurs in other missions as well and whether or not you leave in the new weapons. My tests in Dos were also with this mission, on the exact same level, and in the exact same room, so I'm definitely comparing apples to apples here (i.e. the Driller in this mission isn't simply set to drop shields sometimes whereas it may not be in other missions).

As noted at the top, this interestingly doesn't appear to happen in the 12/11/2016 beta, yet it happened in v0.58.1 and does so again in the 7/30/2017 build.

Provisionally confirmed. I was only able to activate one matcen in the described level, and the drillers it produced worked properly on every try. (It also produced a few non-driller robots.) However, near segment 66 on level 17, I found a preplaced driller that did drop a shield. I will need to rerun this in a debugger to see why it dropped a shield.

That sounds reasonable. (While it might not matter at this point for this bug, in order to have infinite tries with a matcen, you can just save before activating it and reload/retry if you don't get the result you're looking for.)

As noted in the first post, I didn't seem to have this in the 12/11/2016 beta, but I also did not do enough tries to be particularly confident that the issue did not exist in that version.

Just linking to my post in a different thread, since it's relevant to this report. Kp and I discovered that Class 1 Drillers in Descent II have always had a small chance of dropping shield boosts, even in the original MS-DOS engine.